Every test I have been able to find ranks AVG in at least the top half of performers. The only place where the expensive programs occasionally are better is in detecting "wild" viruses, i.e., those that are not yet in the virus definition file. But the newest version of AVG uses (or more accurately, can use) the same heuristic approcah as the expensive guys, so maybe it is improved over the older releases.
I use AVG and it has been good so far. I also know more than a few people who have switched to AVG from Norton and McAfee because they got sick of paying a ton for the program and then having to pay a subscription fee every year for updates.